Organic electroluminescent display device and production method thereof

ABSTRACT

An organic electroluminescent display device has a structure which suppresses a leakage current generated in an organic electroluminescent layer and achieves a low power consumption and excellent light-emitting characteristics. The organic electroluminescent display device includes a first electrode, an organic electroluminescent layer, and a second electrode, stacked in this order on a substrate, wherein the organic electroluminescent layer includes a conductive layer and a light-emitting layer, the conductive layer has a trapezoidal cross section which widens downwardly, and the light-emitting layer covers upper and side surfaces of the conductive layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescent(hereinafter, also referred to as an organic EL) display device and aproduction method thereof. More specifically, the present inventionrelates to an organic EL display device including an organic EL layerwhich is formed by a wet method such as an ink-jet method. The presentinvention further relates to a production method of such an organic ELdisplay device.

2. Description of the Related Art

An organic EL display has been drawing attention as a next-generationflat panel display (FPD) because such a display is excellent invisibility such as a viewing angle and responsiveness and permits lowpower consumption, slim profile, and flexibility of the display itself.Such an organic EL display device is still inferior to a liquid crystaldisplay (LCD) or a plasma display panel (PDP) in technical completenessor standard of industrial infrastructure. Therefore, practical use ofthe organic EL display device is still only loading on car audios orsome mobile information devices. However, the organic EL display deviceis theoretically the most excellent FPD and therefore, future marketexpansion is expected for the display device.

Such an organic EL display device displays images by driving an organicEL panel having a configuration in which a light-emitting organic ELelement is arranged in every pixel. The organic EL element has astructure in which an organic EL layer including a light-emitting layeris interposed between a pair of electrodes at least one of which hastranslucency. When a voltage is applied to the light-emitting layer, theorganic EL element emits light. In addition to the light-emitting layer,a layer whose conductivity is higher than that of the light-emittinglayer, such as a hole injection layer, is normally arranged as theorganic EL layer in order to improve a light-emitting efficiency.

A low-molecular organic EL layer and a polymer organic EL layer areknown as the organic EL layer. A method of forming a film by vacuumdeposition is commonly used to form the low-molecular organic EL layer,but an uneven film tends to be formed by this method. Therefore, it isdifficult to form a large organic EL layer by this method. In contrast,a wet method such as an ink-jet method, a nozzle coating method, and aprinting method is commonly used to form the polymer organic EL layer.Among these, a method in which RGB pixels are pattern-formed on asubstrate by an ink-jet method has been widely used, recently (forexample, refer to Japanese Kokai Publication No. Hei-10-12377, JapaneseKokai Publication No. Hei-10-153967, and Japanese Kokai Publication No.2002-334782).

However, the organic EL element prepared by the wet method generates aleakage current which does not contribute to light emission when avoltage is applied, which adversely influences the light emittingcharacteristics.

For this problem, an organic EL element in which a region where alight-emitting layer is formed is the same as or larger than a regionwhere a hole injection/transport layer is formed is disclosed (forexample, refer to WO 01/074121). Such an organic EL element generates noleak current and has excellent characteristics such as a highlight-emitting efficiency. However, even if the light-emitting layer isformed to completely cover the hole injection/transport layer, a leakagecurrent must be generated. Therefore, there is room for improvement in areduction in power consumption and improvement in light-emittingcharacteristics.

SUMMARY OF THE INVENTION

In order to overcome the problems with the prior art, preferredembodiments of the present invention provide an organic EL displaydevice in which generation of a leakage current in the organic EL layeris suppressed and a power consumption is low, and light-emittingcharacteristics are excellent, and also provide a production method ofsuch an organic EL display device.

The present inventors made various investigations of an organic ELdisplay device which can suppress generation of a leakage current in anorganic EL layer formed by a wet method such as an ink-jet method. Theinventors noted the shapes of a conductive layer and a light-emittinglayer each constituting the organic EL layer. Then, the inventors madethe following discoveries. If the conductive layer and thelight-emitting layer are formed by a wet method, conventionally, a fluidmaterial for the conductive layer contacts with a bank formed around theorganic EL layer to be raised along the bank, and therefore theconductive layer has a larger thickness at the peripheral portion. As aresult, at the peripheral portion, the thickness of the light-emittinglayer formed on the conductive layer becomes thinner. Then, theinventors made the following discoveries. If the organic EL layerincludes a conductive layer having a trapezoidal cross section whichwidens downwardly and a light-emitting layer which covers upper and sidesurfaces of the conductive layer, generation of the leakage current canbe suppressed. As a result, an organic EL display device which has alower power consumption and excellent light-emitting characteristics canbe provided. Thus, the above-mentioned problems have been admirablysolved, leading to completion of preferred embodiments of the presentinvention.

According to a preferred embodiment of the present invention, an organicelectroluminescent display device includes a first electrode, an organicelectroluminescent layer, and a second electrode, stacked in this orderon a substrate, wherein the organic electroluminescent layer includes aconductive layer and a light-emitting layer, the conductive layer has atrapezoidal cross section which widens downwardly, and thelight-emitting layer covers upper and side surfaces of the conductivelayer.

According to the organic EL display device of a preferred embodiment ofthe present invention, the first electrode, the organic EL layer, andthe second electrode are stacked on the substrate in this order. Thatis, according to the configuration of the organic EL display device of apreferred embodiment of the present invention, an organic EL elementwhich includes the first electrode, the organic EL layer, and the secondelectrode is arranged on the substrate. The organic EL layer, and thefirst or second electrode are individually arranged in a pixel,generally. When a current flows into the respective organic EL layers,the light-emitting layer emits light. As a result, an image isdisplayed. The organic EL display device of a preferred embodiment ofthe present invention may be a bottom emission or top emission typeorganic EL display device.

The above-mentioned substrate normally includes: on a glass or resinsubstrate, wirings which are connected to the first and/or secondelectrodes to flow a current into the organic EL layer in each pixel; acircuit element arranged to control an amount of the current; and aninsulating film arranged to electrically separate a layer where thewirings are arranged from a layer where the first electrode is arranged,for example. An active element such as a thin film transistor (TFT) ispreferably used as the above-mentioned circuit element. With regard tothe above-mentioned first and second electrodes, one is an anode and theother is a cathode. At least one of the first and second electrodesnormally has translucency and transmits light emitted from thelight-emitting layer.

If the organic EL display device according to a preferred embodiment ofthe present invention is a bottom emission type, a translucent materialis used for the above-mentioned substrate and the first electrode. Ifthe organic EL display device of a preferred embodiment of the presentinvention is a top emission type, a translucent material is used for thesecond electrode.

In a preferred embodiment of the present invention, the above-mentionedorganic EL layer includes a conductive layer and a light-emitting layer.The conductive layer has a trapezoidal cross section which widensdownwardly. The light-emitting layer covers upper and side surfaces ofthe conductive layer. The conductive layer is not especially limited aslong as it shows a conductivity higher than that of the light-emittinglayer at a normal temperature (25° C.). The conductive layer is alsocalled buffer layer, hole transport layer, hole injection layer,electron transport layer, electron injection layer, and the like,generally. The conductive layer generally has a function of smoothlyinjecting a hole or an electron which has flowed from the firstelectrode, into the light-emitting layer, or a function of flatteningthe substrate surface. In the light-emitting layer, the hole injectedfrom the anode is recombined with the electron injected from thecathode, and thereby light is emitted.

The organic EL layer may be a polymer organic EL layer or alow-molecular organic EL layer. However, in a preferred embodiment ofthe present invention, a polymer organic EL layer is preferable becausesuch a polymer organic EL layer is suitably formed by a wet method. Apolymer organic layer composed of a multilayer of an anode, a conductivelayer, a light-emitting layer, and a cathode is preferably used.

In various preferred embodiments of the present invention, theconductive layer has a trapezoidal cross section which widensdownwardly. In the present description, the trapezoidal cross sectionwhich widens downwardly means a cross section whose bottom surface has awidth larger than a width of its upper surface. Accordingly, the uppersurface may not be parallel to the bottom surface. For example, theconductive layer may have a hemispherical or substantially hemisphericalcross section. The trapezoidal cross section which widens downwardly isformed when the conductive layer is preferably formed in the followingmanner: a droplet of a material for the conductive layer is applied by awet method such as an ink-jet method, and this material is dried andsolidified without excessively being wet and spread. In a preferredembodiment of the present invention, the conductive layer is formed intosuch a shape, and thereby a profile (thickness distribution) of thelight-emitting layer on the conductive layer is controlled. That is,because of the conductive layer having a trapezoidal cross section whichwidens downwardly, the light-emitting layer formed on the conductivelayer can cover the upper and side surfaces of the conductive layer,which can suppress the light-emitting layer from having a portion wherethe thickness is small at the peripheral portion. Thus, according to theorganic EL layer of a preferred embodiment of the present invention, theconductive layer having a high conductivity can be sufficientlyprevented from contacting with the second electrode, and therefore,generation of the leakage current can be suppressed.

According to a preferred embodiment of the present invention, theconductive layer has a trapezoidal cross section which widensdownwardly. According to such a preferred embodiment, the light-emittinglayer can be more surely formed to have a sufficient thickness near theend of the conductive layer when the light-emitting layer is formed onthe conductive layer. Accordingly, the yield of the organic EL elementcan be improved when it is commercially produced, and the organic ELelement can obtain a high productivity. Such an effect is remarkablyexhibited particularly when the light-emitting layer is a layer formedby a wet method.

It can be possible in principle to suppress generation of the leakagecurrent by forming the light-emitting layer to have a larger thickness,thereby increasing a distance between the conductive layer and thesecond electrode. However, if, in an organic EL element having aconventional structure, and the light-emitting layer is formed to have athickness large enough to sufficiently reduce a leakage current, aluminance which is needed in practice is not obtained normally. This isbecause of a trade-off relationship: when the thickness of alight-emitting layer is increased, the lifetime of an organic EL elementis extended but the luminance is reduced. However, according to variouspreferred embodiments of the present invention, the leakage current issufficiently reduced and simultaneously the light-emitting layer can bethinned. Therefore, not only an improvement in light-emittingefficiency, attributed to the reduction in leakage current, but also animprovement in luminance, attributed to the thinning of thelight-emitting layer can be satisfied. Also in such a point, thepreferred embodiments of the present invention is excellent.

A mixture (PEDOT/PSS) of polyethylene dioxythiophene and polystyrenesulfonic acid and the like may be mentioned as a material for theconductive layer. A polyfluorene compound represented by the followingformula (1) and the like may be mentioned as a material for thelight-emitting layer.

The polyfluorene compound represented by the above formula (1) is acopolymer compound of a fluorene ring having alkyl chains R and R′ witha unit of one or more aryl compounds. In the above formula (1), each ofR and R′ represents an alkyl chain; each of Ar and Ar′ represents a unitof an aryl compound; each of l and m is an integer of 1 or more; and nis an integer of 0 or 1 or more. Examples of the aryl compound includedimethylbenzene, pyridine, benzene, anthracene, spirobifluorene,carbazole, benzo amine, bipyridine, and benzothiadiazole. Theabove-mentioned polyfluorene compound preferably has hundreds ofthousands of weight average molecular weights. A color emitted dependson a unit to be used for copolymerization and a ratio among l, m, and n.

The above-mentioned organic EL layer may include an intermediate layer.The intermediate layer is not especially limited as long as it has afunction of preventing an electron which has been transported from thecathode through the light-emitting layer from being injected into theconductive layer. The formation of the intermediate layer extends thelifetime of the organic EL layer, normally. The organic EL layerincluding the intermediate layer may be composed of a multilayer of ananode, a conductive layer, an intermediate layer, a light-emittinglayer, and a cathode. It is preferable that the intermediate layer has aconductivity equivalent to the conductivity of the light-emitting layer.Examples of a material for the intermediate layer include:

-   poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-ethylenylbenzene)],    poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(9,ethyl-3,6-carbazole)],    and poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(2,5-p-xylene)]. Such a    compound is dissolved into a nonpolar solvent to provide a liquid    material for forming the intermediate layer. It is preferable that    the thickness of the intermediate layer is half or less than the    thickness of the light-emitting layer.

Preferred embodiments of the organic EL display device of the presentinvention are mentioned in more detail below.

It is preferable that the organic EL display device includes a bankaround the organic electroluminescent layer. It is more preferable inthe organic EL display device that the light-emitting layer is incontact with a wall surface of the bank. In the present description, thebank is not especially limited as long as it is a structure forseparating the organic layers from each other. Due to the formation ofsuch a bank, a fluid material for the organic EL layer can be maintainedinside the region surrounded by the bank if the organic EL layer isformed by a wet method. In each pixel, the organic layer can be formedinto a certain planar shape. In contrast, according to a conventionalconfiguration, if the bank is formed around the organic EL layer, theorganic EL material is wetted and spread along the bank, and as aresult, the conductive layer at the peripheral portion has a thicknesslarger than a thickness at the center. Further, the light-emitting layerformed on the conductive layer has a smaller thickness at the peripheralportion. In contrast, according to a preferred embodiment of the presentinvention, the conductive layer is formed to have a trapezoidal crosssection which widens downwardly by, for example, a method of controllinglyophilic and lyophobic properties of the bank surface. Accordingly, thelight-emitting layer can be formed to have a large thickness at theperipheral portion and therefore the leakage current generated in theorganic EL layer can be effectively suppressed. An insulating organicsubstance such as polyimide may be mentioned as a material for the bank.

It is preferable that the light-emitting layer has an H-shaped crosssection. In the present description, the “H-shaped cross section” is across section both ends of which upwardly and downwardly project fromthe center portion. When the bank is arranged around the organic ELlayer and the light-emitting layer is formed by a wet method such as anink-jet method, the material for the light-emitting layer is wetted andspread along the bank, and therefore, the light-emitting layer is formedon the wall surface of the bank. As a result, the light-emitting layerhas an H-shaped cross section. The thickness at the peripheral portionis sufficiently large because the light-emitting layer has such a shape.Thus, a layer having a relatively large conductivity can be formed tohave a certain or larger thickness between the conductive layer and thecathode. As a result, the leakage current in the organic EL layer can bemore effectively reduced.

Another preferred embodiment of the present invention provides aproduction method of an organic electroluminescent display device, theorganic electroluminescent display device including a first electrode;an organic electroluminescent layer including a conductive layer and alight-emitting layer; the second electrode, stacked on a substrate inthis order; and a bank around the organic electroluminescent layer,wherein the production method includes the steps of, in the followingorder: (lyophilic and lyophobic properties-giving step) subjecting asurface of the first electrode to a lyophilic treatment and subjecting asurface of the bank to a lyophobic treatment; (conductive layer-formingstep) forming the conductive layer on the first electrode by a wetmethod; (step of giving a lyophilic property to the bank) reducing alyophobic property on the surface of the bank; and (light-emittinglayer-forming step) forming the light-emitting layer on the conductivelayer by a wet method. According to such a production method of theorganic EL display device of a preferred embodiment of the presentinvention, the conductive layer can be formed to have a small thicknessand the light-emitting layer can be formed to have a large thickness atthe portion near the bank. As a result, the leakage current in theorganic EL layer can be reduced. Accordingly, such a method ispreferably employed to produce the organic EL display device of apreferred embodiment of the present invention.

In the above-mentioned lyophilic and lyophobic properties-giving step, alyophilic treatment is performed for the first electrode surface and alyophobic treatment is performed for the bank surface. The methods forthe lyophilic treatment and the lyophobic treatment are not especiallylimited. A plasma treatment and the like may be mentioned, for example.

In the above-mentioned conductive layer-forming step, a conductive layeris formed on the first electrode by a wet method. When a conductivelayer is formed by a wet method, the first electrode surface tends toshow an affinity to a liquid material for the conductive layer, but thebank surface tends to repel the liquid material for the conductivelayer, because the first electrode surface is provided with thelyophilic property and the bank surface is provided with the lyophobicproperty by the above-mentioned lyophilic and lyophobicproperties-giving step. As a result, the conductive layer tends to beformed to have a uniform thickness on the first electrode surface.Further, the conductive layer is formed to have a small thickness on thebank or not formed on the bank. Examples of the wet method employed forforming the conductive layer include: coating methods such as a spincoating method, an ink-jet method, a nozzle coating method, a slitcoating method, a die coating method; and printing methods such as anoffset printing method and an intaglio printing method; and a lasertransfer method. Among these, coating by an ink-jet method and heatingor natural drying are preferably used in combination.

In the above-mentioned step of giving a lyophilic property to the bank,the lyophobic property on the bank surface is reduced. As a method ofreducing a lyophobic property on the bank surface, a rinse treatmentusing a solvent is preferably used, for example. That is, it ispreferable that the step of reducing a lyophobic property is a step oftreating the surface of the bank with a solvent. According to the methodof treating the bank surface with a solvent, the lyophobic property onthe bank surface can be easily reduced. The solvent is not especiallylimited as long as it can reduce the lyophobic property on theconductive layer surface. A solvent which does not dissolve theconductive layer is preferable. An aromatic solvent such as toluene,anisole, and xylene is preferably used if the conductive layer is madeof a mixture of PEDOT and PSS.

In the above-mentioned light-emitting layer-forming step, thelight-emitting layer is formed on the conductive layer by a wet method.When the light-emitting layer is formed by a wet method, the banksurface tends to show an affinity to a liquid material for thelight-emitting layer more strongly than that to the liquid material forthe conductive layer because the lyophobic property on the bank surfaceis reduced by the above-mentioned step of giving a lyophilic property tothe bank. Accordingly, the light-emitting layer can be formed to have alarge thickness at the portion near the bank. As a result, thelight-emitting layer can be formed in the entire space between theconductive layer below the light-emitting layer and the second electrodeabove the light-emitting layer to have a thickness necessary to suppressthe leakage current. The organic EL display device including thethus-prepared organic EL layer has a low power consumption and excellentlight-emitting characteristics because the leakage current can beeffectively reduced. The same methods as the wet method employed forforming the conductive layer can be employed for forming thelight-emitting layer.

According to various preferred embodiments of the present invention, theconductive layer constituting the organic EL layer has a trapezoidalcross section which widens downwardly and the light-emitting layer has ashape covering the upper and side surfaces of the conductive layer. As aresult, the light-emitting layer having a low conductivity can be formedin the entire space between the conductive layer and the electrode, andtherefore generation of the leakage current in the organic EL layer canbe suppressed. Accordingly, an organic EL display device which has a lowpower consumption and excellent light-emitting characteristics can beprovided.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1C are cross-sectional views schematically showing the organicEL element in Preferred Embodiment 1, wherein FIG. 1A shows the entirestructure of the organic EL element, FIG. 1B shows the thickness profileof the conductive layer, and FIG. 1C shows the thickness profile of thelight-emitting layer.

FIG. 2 is a planar view schematically showing the organic EL displaydevice in Preferred Embodiment 1.

FIGS. 3A-3C are cross-sectional views schematically showing the organicEL element in Preferred Embodiment 2, wherein FIG. 3A shows the entirestructure of the organic EL element, FIG. 3B shows the thickness profileof the conductive layer, FIG. 3C shows the thickness profile of thelight-emitting layer.

FIGS. 4A-4C are cross-sectional views schematically showing the organicEL element in Comparative Preferred Embodiment 1, wherein FIG. 4A showsthe entire structure of the organic EL element, FIG. 4B shows thethickness profile of the conductive layer, FIG. 4C shows the thicknessprofile of the light-emitting layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in more detail below with referenceto preferred embodiments using drawings, but the present invention isnot limited to these Embodiments.

Preferred Embodiment 1

FIG. 1 is a cross-sectional view schematically showing an organic ELelement in accordance with Preferred Embodiment 1. As shown in FIG. 1A,the organic EL element in the present preferred embodiment has astructure in which an organic EL layer 5 a including a conductive layer3 a and a light-emitting layer 4 a is interposed between an anode (thefirst electrode) 1 and a cathode (the second electrode) 2. Theconductive layer 3 a has a trapezoidal cross section which widensdownwardly and has a thickness of about 60 nm at the center, as shown inthe profile and thickness of the coating film in FIG. 1B. Thelight-emitting layer 4 a has an H-shaped cross section and covers theupper and side surfaces of the conductive layer 3 a. As shown in theprofile and thickness of the coating film in FIG. 1C, the light-emittinglayer 4 is preferably formed to have a thickness of about 80 nm or more,for example, over the entire conductive layer 3 a.

FIG. 2 is a planar view schematically showing an organic EL displaydevice in accordance with Preferred Embodiment 1. As shown in FIG. 2,the organic EL display device in the present preferred embodiment has astructure in which organic EL elements having the cross-sectionalconfiguration shown in FIG. 1 are partitioned by a bank 6 to be arrangedin a matrix pattern. The bank 6 is arranged around the organic EL layer5 a to cover the outer edge of the anode 1.

The organic EL display device in Preferred Embodiment 1 was produced asfollows.

“Lyophilic and Lyophobic Properties-Giving Step”

A substrate on which the anodes 1 and the bank 6 were arranged wasprepared in the following manner, first. The anodes 1 made of indium tinoxide (ITO) were arranged in a matrix pattern. The bank 6 which includedelliptical openings each having a long axis of about 180 μm and a shortaxis of about 60 μm, for example, was formed in a region except for theregion where the anode 1 had been formed. Then, as a pretreatment beforeforming the organic EL layer 5 a in the openings of the bank 6 on thesubstrate, a plasma treatment was performed using a plasma treatmentapparatus (product of PVA TePla America, Inc., trade name: Plasma System400). Specifically, an oxygen plasma treatment and oxygen-CF₄ plasmatreatment were successively performed for the anode 1 surface and thebank 6 surface, thereby providing the anode 1 surface with a lyophilicproperty and providing the bank 6 surface with a lyophobic property.After the plasma treatments, the anode 1 surface had a water contactangle of about 10° and the bank 6 surface had an anisole contact angleof about 75°, for surface. The conditions for the plasma treatments areshown in the following Table 1. In Table 1, step 1 represents the oxygenplasma treatment and step 2 represents the oxygen-CF₄ plasma treatment.

TABLE 1 Flow Electrical rate Pressure power Time Gas (sccm) (m bar) (W)(s) Step 1 O₂ 500 0.545 900 120 Step 2 CF₄/O₂ 100/30 0.257 900 60“Conductive Layer-Forming Step”

A coating liquid for forming the conductive layer was prepared. Thecoating liquid included the following components at the followingproportions.

PEDOT/PSS (product of 1 part by weight H. C. Starck-VTech Ltd., tradename: Baytron P CH8000) Water 3 parts by weight Ethanol 4 parts byweight Ethylene glycol 2 parts by weight

The coating liquid for forming the conductive layer, which included theabove components at the above proportions, was applied, by an ink-jetmethod, to the openings of the bank 6 for which the plasma treatment hadbeen performed. Then, the coating liquid was baked at 200° C. for 60minutes to form the conductive layer 3 a. The formed conductive layer 3a had a trapezoidal cross section widening downwardly and it had athickness of about 60 nm at the center.

“Step of Giving a Lyophilic Property to the Bank”

The substrate on which the conductive layer 3 a had been formed wasimmersed into anisol for about 2 minutes and then air-dried. Then, thesubstrate was subjected to baking at 200° C. for 10 minutes. Attributedto this solvent rinse treatment using anisol, the lyophobic property onthe bank 6 surface was reduced and the anisol contact angle on the bank6 surface was decreased from about 75° to about 50°, for example.

“Light-Emitting Layer-Forming Step”

A coating liquid for forming the light-emitting layer was prepared. Thecoating liquid including the following components at the followingproportions was prepared. A polyfluorene compound represented by thefollowing formula (1) was used as a green light-emitting polymermaterial.

Green light-emitting polymer material  1 part by weight Xylene 60 partsby weight Tetralin 60 parts by weight Formula 2

(1)

According to the green light-emitting polymer material, in the aboveformula (1), each of R and R′ to which a fluorene ring is bondedrepresents an alkyl chain; each of Ar and Ar' represents an arylcompound unit; each of l and m is an integer of 1 or more; and n is aninteger of 0 or 1 or more.

The green light-emitting polymer material had hundreds of thousands ofweight average molecular weights.

The coating liquid for forming the light-emitting layer, which includedthe above components at the above proportions, was applied to theopenings of the bank 6 where the conductive layer 3 a had been formedand for which the step of giving a lyophobic property to the bank hadbeen performed by an ink-jet method. Then, the coating liquid was bakedat 200° C. for 60 minutes under nitrogen atmosphere. As a result, alight-emitting layer 4 a was formed. The formed light-emitting layer 4 ahad an H-shaped cross section as shown in FIG. 1( c) and it had thesmallest thickness of about 80 nm, for example, at the center. In thepresent preferred embodiment, the conductive layer 3 a had a trapezoidalcross section widening downwardly. Therefore, the light-emitting layer 4a which had a shape covering the entire conductive layer 3 a and whichhad a thickness of about 80 nm or more, for example, could be easilyformed. As a result, the light-emitting layer 4 a having a thickness ofabout 80 nm or more, for example, exists between the conductive layer 3a and the cathode 2, and therefore the leakage current can be reduced.

“Cathode-Forming Step and Successive Steps”

After the light-emitting layer 4 a was formed, calcium and silver weredeposited on the light-emitting layer 4 a and the bank 6 by a vacuumdeposition method. As a result, the cathode 2 was formed. Then, theregion where the organic EL element had been formed on the substrate wassealed with a glass cap under nitrogen atmosphere. As a result, anorganic EL display device was completed.

Preferred Embodiment 2

In Preferred Embodiment 2, an organic EL display device was produced inthe same manner as in Preferred Embodiment 1, except that a coatingliquid for forming the conductive layer which contained ethylenecarbitol 2 parts instead of the ethylene glycol was used in theconductive layer-forming step. FIG. 3 is a cross-sectional viewschematically showing an organic EL element in Preferred Embodiment 2.As shown in FIG. 3A, the formed conductive layer 3 b had a U-shapedcross section both ends of which had a larger thickness. Thelight-emitting layer 4 b had a U-shaped cross section both ends of whichhad a larger thickness. In the present preferred embodiment, the bank 6was provided with the lyophobic property-reducing treatment and then thelight-emitting layer 4 b was formed. Therefore, an area of a regionwhere the light-emitting layer 4 b adhered to the bank 6 was larger thanan area of a region where the conductive layer 3 b adhered to the bank6. The conductive layer 3 b had a thickness of 60 nm at the center, asshown in FIG. 3B. The light-emitting layer 4 b covered only the uppersurface of the conductive layer 3 b, and as shown in FIG. 3C, it had thesmallest thickness of about 80 nm, for example, at the center. In thepresent preferred embodiment, the light-emitting layer 4 b which coveredthe entire conductive layer 3 b and preferably had a thickness of 80 nmor more, for example, could be easily formed. As a result, thelight-emitting layer 4 b having a thickness of about 80 nm or more, forexample, exists between the conductive layer 3 b and the cathode 2, andtherefore the leakage current can be reduced.

Comparative Preferred Embodiment 1

In Comparative Preferred Embodiment 1, an organic EL display device wasproduced in the same manner as in Preferred Embodiment 2, except thatthe step of giving a lyophobic property to the bank was omitted. Asshown in FIG. 4A, the formed conductive layer 3 c had a U-shaped crosssection both ends of which had a larger thickness. In contrast, alight-emitting layer 4 c had a U-shaped cross section both ends of whichhad a smaller thickness and the light-emitting layer 4 c covered onlythe upper surface of the conductive layer 3 c because the lyophobicproperty-reducing treatment was not performed for the bank beforeforming the light-emitting layer 4 c. The conductive layer 3 c had athickness of about 60 nm at the center, as shown in FIG. 3B. Incontrast, the light-emitting layer 4 c covered the entire conductivelayer 3 c, as shown in FIG. 4A, but it had the smallest thickness ofabout 80 nm or less at the both ends, as shown in FIG. 3C. At such aportion where the light-emitting layer 4 c has a small thickness, thedistance between the conductive layer 3 c and the cathode 2 c is small,and therefore the leakage current tends to be generated.

“Evaluation Test”

The organic EL display devices in Preferred Embodiments 1 and 2 andComparative Embodiment 1 were measured for a current density and alight-emitting efficiency. The current density is a value obtained whena voltage of 1.5 V was applied to the organic EL element. Thelight-emitting efficiency was a value measured when the organic ELelement emitted light at a luminance of 300 cd/m². Table 2 shows theresults.

TABLE 2 Current density Light-emitting efficiency (mA/^(cm2)) (cd/A)Example 1 0.000012 11.5 Example 2 0.000011 11.3 Comparative 0.034 6.7Example 1

As clearly shown in the above Table 2, each of the organic EL displaydevices in Preferred Embodiments 1 and 2 showed a smaller currentdensity and a higher light-emitting efficiency, than those of theorganic EL display device in Comparative Example 1. Accordingly, theeffect of reducing the leakage current, attributed to preferredembodiments of the present invention, could be determined. The resultsof the evaluation test shows that the organic EL display devices inPreferred Embodiments 1 and 2 are equivalent in terms of light-emittingcharacteristics. However, if it is taken into consideration that thethickness profile of the conductive layer is changed when unintendedchanges of the production conditions occur, the conductive layer inPreferred Embodiment 1 is more suitable for suppressing a shortage inthickness of the light-emitting layer at the both ends, than theconductive layer in Preferred Embodiment 2 has. The conductive layer inPreferred Embodiment 1 is more excellent than that in PreferredEmbodiment 2 because the thickness of the light-emitting layer can bemore surely maintained to a predetermined value or more over the entireconductive layer. That is, if the organic EL display device iscommercially produced, the thickness profile in Preferred Embodiment 1is excellent in terms of reduction in leakage current.

The present application claims priority under the Paris Convention andthe domestic law in the country to be entered into national phase onPatent Application No. 2006-104281 filed in Japan on Apr. 5, 2006, theentire contents of which are hereby incorporated by reference.

In the present description, if the term “or more” is used, the valuedescribed (boundary value) is included.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A production method of an organic electroluminescent display deviceincluding a first electrode, an organic electroluminescent layerincluding a conductive layer and a light-emitting layer, a secondelectrode, stacked on a substrate in this order, and a bank around theorganic electroluminescent layer, the production method comprising thesequentially performed steps of: subjecting a surface of the firstelectrode to a lyophilic treatment and subjecting a surface of the bankto a lyophobic treatment; forming the conductive layer on the firstelectrode by a wet method; reducing a lyophobic property on the surfaceof the bank; and forming the light-emitting layer on the conductivelayer by a wet method.
 2. The production method of the organicelectroluminescent display device according to claim 1, wherein the stepof reducing the lyophobic property is a step of treating the surface ofthe bank with a solvent.